In optical transmission schemes and systems of the above-mentioned types Raman fibre amplifiers are generally used to amplify the propagated optical signals. The Raman amplifiers are generally designed to operate at a substantially flat spectral gain referred to as “nominal gain”. Particularly in cases which lead to a change in the number of wavelength channels operable in the optical transmission system, so-called “channel adds” or “channel drops”, e.g. due to fibre break or the like, a change in gain may be encountered.
Due to various interactions between different wavelength channels, in particular in dense wavelength division multiplexing (DWDM) systems such a change in channel number will generally lead to a transfer of optical power between different wavelength channels. However, in order to keep an appropriate bit error ratio (BER) for the surviving channels, their respective Raman gain must be maintained at essentially said nominal value. To this end, the pump powers of individual pump light sources comprised in the Raman pump source are tuned, i.e. adjusted in such a way as to produce roughly nominal valued Raman gain on surviving channels, i.e. a channel operable before and after a channel add/drop.
In prior art document EP 1 130 819 A1, a method for determining a wavelength-dependent gain figure of an optical amplifier is described, including a measurement of the total amplifier gain through measuring total input and output power of the amplifier. A gain tilt of the determined gain figure is compensated for by application of a gain tilt of opposite sign and magnitude in order to generate a flat gain figure over the amplified wavelength band.
This prior art approach suffers from the disadvantage, that in actual optical transmission systems of the above-mentioned type an optical output power on an existing/surviving channel may change abruptly during short periods of time depending on a position of said channels in the optical bandwidth of the transmission system before finally changing in the other direction due to a complex and temporarily diversified combination of signal-signal interactions (SSI) and signal-pump interactions (SPI). Thus, when dealing with channel adds/drops the above-described prior art approach may actually lead to an inadequate Raman gain control which will in turn deteriorate overall system performance.